Polyolefins are useful in any number of everyday articles. For many of these applications, including films and fibers, flexibility and softness combined with retention of properties at high end-use temperatures (“thermal resistance”) are desirable attributes. Semicrystalline polyolefins are used in such applications because they are thermoplastic, meaning the crystalline nature (not cross-linking) allows them to form useful articles; such materials can be formed into pellets for ease of handling and are processed using standard plastic-industry equipment such as extruders. This is in contrast to polyolefin thermoset materials, such as ethylene-propylene-diene (EPDM) elastomer (also called EPDM rubber), which have very little to no crystallinity and very high molecular weights, and must be highly cross-linked to form useful articles; such materials can not be pelletized and are processed using standard rubber-industry equipment such as roll mills.
Often, propylene-based semicrystalline polyolefins are chosen for their relatively high melting temperature. However, one drawback to such polyolefins, especially propylene-rich ones, is their high hardness and stiffness which makes then unsuitable for applications requiring a soft touch. Another drawback to propylene-rich polyolefins is their relatively high glass transition temperature, which is detrimental to toughness, particularly low temperature toughness and impact resistance, that is often critical to applications that involve structural parts fabricated by molding or extrusion techniques.
One way of making semicrystalline polypropylene softer and tougher has been to blend in softer polymers such as elastomers and plastomers. This, however, leads to hazy optical properties. In addition, these compositions, while exhibiting soft touch, do not exhibit any elastic properties such as good compression set and elastic recovery after high strains. This characteristic makes these polyolefins incapable of use in a wide variety of applications where softness and elasticity are required along with good thermal resistance. Another common approach to improving flexibility, softness, toughness and/or elasticity of semicrystalline polypropylene is to lower the polymer crystallinity by addition of comonomer. However, this results in reduced melting points, and therefore lower thermal resistance and possibly poor pellet quality. It is often desirable for a polyolefin to exhibit good pellet-forming and pellet-stability characteristics to ease use of the polyolefin in processing operations.
Many applications of semicrystalline polyolefins benefit from having useful properties over a broad range of temperatures; consequently, there is a need to provide polyolefins that can maintain desirable characteristics such as high Vicat softening temperature, while maintaining or improving upon the softness and elasticity. In particular, it would be advantageous to provide a propylene-based polymer composition possessing improved softness, toughness, elasticity and clarity, without sacrificing its other desirable properties. Specifically, what is needed is a composition that exhibits excellent softness, toughness and/or elasticity, without sacrificing thermal resistance, clarity and/or pellet quality. A modified semicrystalline polyolefin according to this invention fulfills these needs.
A low melt viscosity (high melt flow rate) is advantageous for almost all polyolefin fabrication processes, because this reduces cycle time or allows for lower temperature and/or energy requirements. Traditional approaches to achieve low melt viscosity are lowering the molecular weight and broadening the molecular weight distribution of the resin. However, both approaches can have detrimental effects on the final physical properties of the polyolefin article due to the presence of low molecular weight polymer. Therefore, what is further needed is a method to improve physical properties, as described above, while simultaneously lowering melt viscosity. Moreover, it would also be advantageous in a fabrication environment be able to continuously vary these parameters to match changing needs, instead of choosing between discrete grades of polyolefin(s) and/or blending different polyolefins which requires great expertise and care to properly control the morphology and final properties of the blend. A modified semicrystalline polyolefin composition according to this invention fulfills these needs.
Addition of a low molecular weight, amorphous substance to a semicrystalline polyolefin is one way to attempt to address the above needs. Some patent disclosures directed to such an end are U.S. Pat. Nos. 3,201,364, 3,415,925, 4,073,782, 4,110,185, 4,132,698, 4,210,570, 4,325,850, 4,960,820, 4,774,277, 5,869,555, 6,465,109, EP 0448259, FR 2094870, and JP 09208761. These disclosures are directed to semicrystalline polyolefins blended with materials such as mineral oils which often contain substantial concentrations of unsaturation, aromatic groups, naphthenic groups, and/or other functional groups. Addition of mineral oils in polyolefin elastomers, which have little to no crystallinity and very high molecular weights, is also well known; see e.g., RUBBER TECHNOLOGY HANDBOOK, Werner Hoffman (Hanser, N.Y., 1989), p. 294-305.
Addition of mineral oils tend to improve the flexibility of a semicrystalline polyolefin, which identifies such compounds as “plasticizers” under the commonly accepted definition; that is, a substance that improves the flexibility, workability, or distensibility of a plastic or elastomer. Mineral oils are also added to polyolefins as extender oils or processing oils, as well as for other purposes. However, use of these additive compounds typically does not preserve the optical properties (e.g., color and/or transparency) or low odor of the polyolefin, among other things. The melting point of the polyolefin is also typically not preserved, which reduces the softening point and upper use temperature of the composition. In addition, such additive compounds often have high pour points (greater than −20° C., or even greater than −10° C.), which results in little or no improvement in low temperature properties or impact toughness of the polyolefin, especially when the glass transition temperature is not lowered.
The addition of mineral oils often translates into a lower melt viscosity and improved processibility of the polyolefin composition. Unfortunately, this often leads to other problems. For example, all or some of the additive can migrate to a surface and evaporate at an unacceptably high rate, which results in deterioration of properties over time. If the flash point is sufficiently low (e.g., less than 200° C.), the compound can cause smoking and be lost to the atmosphere during melt processing. It can also leach out of the polyolefin and impair food, clothing, and other articles that are in contact with the final article made from the polyolefin composition. It can also cause problems with tackiness or other surface properties of the final article. What is needed is a compound which imparts superior low temperature properties while also exhibiting low bloom, migration, leaching, and/or evaporation behaviors.
Another shortcoming of typical additive compounds is that they often contain a high (greater than 5 wt %) degree of functionality due to carbon unsaturation and/or heteroatoms, which tends to make them reactive, thermally unstable, and/or incompatible with polyolefins, among other things. Mineral oils, in particular, consist of thousands of different compounds, many of which are undesirable for use in polyolefins due to molecular weight or chemical composition. Under moderate to high temperatures these compounds can volatilize and oxidize, even with the addition of oxidation inhibitors. They can also lead to problems during melt processing and fabrication steps, including degradation of molecular weight, cross-linking, or discoloration. They may also impart an undesirable odor.
These attributes of common additive compounds like mineral oils limit the performance of the final polyolefin composition, and therefore its usefulness in many applications. As a result, they are not highly desirable for use as modifiers for semicrystalline polyolefins. What is needed is a modifier that does not suffer from these deficiencies. Specifically, what is needed is a modifier that allows the formulation of semicrystalline polyolefin compositions with improved softness, flexibility (lower flexure modulus), and impact toughness especially at low temperatures (below 0° C.), while not materially degrading thermal resistance and with minimal migration of low molecular weight substances to the surface of fabricated articles. Ideally, the modifier has a low pour point, while still of sufficient molecular weight to avoid unacceptable exudation and extraction. It should also not contribute to deterioration of optical properties, color, smell, thermal stability, oxidative stability, and the like. Preferably, the glass transition temperature of the modified polyolefin composition is lower than that of the unmodified polyolefin. Modifiers and modified semicrystalline polyolefin compositions according to this invention fulfill these needs.
It would be particularly desirable to modify semicrystalline propylene-rich polyolefins by addition of a simple, non-reactive compound such as a paraffin liquid. However, it has been taught that addition of aliphatic or paraffinic compounds impairs the properties of polyolefins, and is thus not recommended; see, e.g., CHEMICAL ADDITIVES FOR PLASTICS INDUSTRY (1987, Radian Corp., Noyes Data Corporation, NJ), p. 107-116. Other background references of interest include U.S. Pat. No. 6,639,020 and ADDITIVES FOR PLASTICS, J. Stepek, H. Daoust (Springer Verlag, New York, 1983), p. 6-69.
Examples of semicrystalline propylene-based polyolefins combined with paraffinic liquid plasticizers for non-adhesive applications include the following. However, none of these disclosed compositions speak to a balance of good thermal resistance, softness, toughness and/or elasticity; more specifically, none speak to elasticity as measured by compression or tension set.
U.S. Pat. No. 4,536,537 discloses polypropylene compositions that comprise LLDPE having a density of 0.912 to 0.935 g/cm3 or polybutene and poly-α-olefin liquid having a kinematic viscosity of about 2 cSt to about 6 cSt at 100° F./38° C.; those with viscosity greater than about 2 cSt are reported to “not work” (col 3, In 12).
WO 98/44041 discloses blend compositions that comprise a chlorine-free polyolefin and poly-α-olefin oligomers having a kinematic viscosity at 100° C. of about 4 cSt to about 8 cSt for a sheet-like structure, especially a floor covering.
WO 2002/18487 and WO 2003/48252 disclose polypropylene compositions that comprise 10 to 30 wt % of vulcanized or unvulcanized polyolefin elastomers, especially EPDM or styrene-ethylene-butene-styrene (SEBS) block-copolymers, and poly-α-olefin oligomers having a kinematic viscosity at 100° C. of about 4 cSt to about 8 cSt.
U.S. Pat. No. 4,645,791, JP 07292167, EP 0315363, and WO 2002/31044 all disclose poly-α-olefin type materials in EPDM compositions.
JP 56095938 discloses polypropylene compositions that comprise olefin oligomer plasticizers mixed with polyolefin granules.
WO 2004/14998 discloses propylene-based polymer compositions that comprise various non-functionalized plasticizers.
Other references of interest include: GB 1329915, JP 01282280, JP 69029554, WO 2001/18109, EP 0300689, EP 1028145.
Certain mineral oils have been classified as Hydrocarbon Basestock Group I, II, or III by the American Petroleum Institute (API) according to the amount of saturates and sulfur they contain and their viscosity indices. Group I basestocks are solvent-refined mineral oils that contain the highest levels of unsaturates and sulfur, and low viscosity indices; they tend to define the bottom tier of lubricant performance. They are the least expensive to produce and currently account for the bulk of the “conventional” basestocks. Groups II and III basestocks are more highly refined (e.g., by hydroprocessing) than Group I basestocks, and often perform better in lubricant applications. Group II and III basestocks contain less unsaturates and sulfur than the Group I basestocks, while Group III basestocks have higher viscosity indices than the Group II basestocks do. Additional API basestock classifications, namely Groups IV and V, are also used in the basestock industry. Rudnick and Shubkin in SYNTHETIC LUBRICANTS AND HIGH-PERFORMANCE FUNCTIONAL FLUIDS, 2nd Ed. (Marcel Dekker, New York, 1999) describe the five basestock Groups as typically being:    Group I—mineral oils refined using solvent extraction of aromatics, solvent dewaxing, hydrofining to reduce sulfur content to produce mineral oils with sulfur levels greater than 0.03 weight %, saturates levels of 60 to 80 weight % and a Viscosity Index (VI) of about 90;    Group II—mildly hydrocracked mineral oils with conventional solvent extraction of aromatics, solvent dewaxing, and more severe hydrofining to reduce sulfur levels to less than or equal to 0.03 weight % as well as removing double bonds from some of the olefinic and aromatic compounds, saturate levels are greater than 95-98 weight % and VI is about 80-120;    Group III—severely hydrotreated mineral oils with saturates levels of some oils virtually 100%, sulfur contents are less than or equal to 0.03 weight % (preferably between 0.001 and 0.01 weight %) and VI is in excess of 120;    Group IV—“polyalphaolefins,” which are hydrocarbon liquids manufactured by the catalytic oligomerization of linear alpha-olefins having 6 or more carbon atoms; in practice, however, this Group is generally thought of as synthetic basestock fluids produced by oligomerizing alpha-olefins have 4 or more carbons; and    Group V—esters, polyethers, polyalkylene glycols, and includes all other basestocks not included in Groups I, II, III, and IV.
Prior attempts of adding mineral oils to polyolefins to modify properties involve for the most part addition of Group I and Group II mineral oils. Even in cases where the mineral oil is not identified by an API Group classification, such as the case for so-called “process oils,” “technical white oils,” “food grade oils,” etc., such mineral oils are still readily categorized into two classes based on VI alone: those with VI less than 120 (similar to Group I and Group II mineral oils), and those with VI of 120 or greater. Certain aspects of the present invention ideally pertain to substances with a VI of 120 or greater, which excludes Group I and Group II mineral oils and any other mineral oils with VI<120.